Several years ago I visited the cattle country in Brazil.  From the time they turned the bull in until the steer went to market was 7 years. Not sure of the grass quality, but quantity was plentiful.  How many acres of good US grass land would it take to maintain US fed cattle numbers on a grass fed system?  I can put  a thousand pounds on a Holstein steer with about 2/3 acre of corn in a year (starting at 400#), and I'm sure the colored cattle are more efficient than that.